Multifocal contact lenses of the hard type have been available for a number of years, while soft multifocal lenses have been under investigation and development. Multifocal contact lenses, most usually bifocal lenses, are designed to correct presbyopia, a condition which develops with advancing age and causes the eye to lose its ability to focus at near distances such as the normal reading distance. Contact lenses designed to correct presbyopia are generally of two types designated as concentric and nonconcentric or segmented.
The concentric contact lens is characterized in that a material having a different refractive index than that of the principle material surrounds the geometric center of the lens in a generally concentric ring. Alternatively, a portion of the lens may be ground to a shorter focal length in a generally concentric pattern relative to the geometric center of the lens. The concentric contact lens is intended to remain centered on the cornea at all times. Distance vision is obtained through the center portion of the lens which may have a diameter of from 1 to about 4 mm. Near vision is obtained through the peripheral concentric portion of the lens. While a portion of the actual perceived image is always out of focus in such a bifocal lens, the user is generally able to mentally reject the out-of-focus portion of the image in favor of the in-focus portion.
The nonconcentric or segmented contact lens is generally characterized in that the near vision element having a different refractive index or ground to provide a shorter focal length, generally referred to as the bifocal segment of the lens, is located in the lower sector or portion of the lens away from the geometric center which comprises the distance vision portion of the lens. Most segmented contact lenses are intended to translate, i.e., move vertically relative to the pupil of the eye when shifting between the distance vision mode and the near vision mode. Such lenses have an advantage in providing a greater proportion of in-focus image at both far and near distances, but have a disadvantage in that the lens must be designed for controlled translation and for maintaining translation and vertical orientation during use.
Segmented bifocal lenses and other contact lenses which require a predetermined orientation on the eye, such as the toric lens which is intended to correct astigmatism, have commonly utilized two basic techniques to assure correct orientation. The lens may be provided with a base-down prism to increase the mass of the lower portion of the lens and create a weighting effect to orient the lens. The lens may also be provided with horizontal truncation or beveling along the lower and/or upper edges so that the combination of eyelid forces and scleral shaping effectively prevent the lens from rotating on the cornea.
It is an object of the present invention to provide a segmented, soft, multifocal contact lens of improved design. It is a further object to provide an improved soft, multifocal contact lens which is readily translated by the user and is free from distortion in both near and distance visual modes. It is a yet further object of this invention to provide a soft contact lens with improved tear distribution and concomitant oxygen supply to the surface of the eye covered by the lens.